Quorum sensing is a cell-to-cell communication system that permits members of a bacterial population to coordinate their behavior dependent on cell density. The mediators of quorum sensing are small, diffusible pheromones or autoinducers that signal gene expression programs at a sufficient bacterial density. The list of bacterial pathogens that use this method of communication to regulate host colonization and virulence is expanding and includes some of the most common pathogens of humans. We hypothesized that several mammalian effectors contribute to innate immunity by inhibiting pathogenic bacterial communication via "quorum quenching." In preliminary data we show that phagocyte-derived reactive oxygen and nitrogen intermediates (ROI and RNI) target a virulence-inducing peptide of the medically important human pathogen Staphylococcus aureus as an innate defense mechanism of the host. In addition, enzymes in blood and epithelial cells that cleave lactone bonds that are present in these autoinducers could inhibit quorum sensing-dependent virulence. To test this hypothesis, we will pursue the following specific aims: 1) To determine the contribution of phagocyte-derived ROI and RNI to host defense against infection with quorum sensing-sufficient and -deficient strains of S. aureus and S. epidermidis and the ability of ROI and RNI to functionally inactivate the virulence pheromones secreted by these pathogens; 2) To determine the ROI- and RNI-mediated modifications of these virulence pheromones by mass spectrometry in vitro and their biologic significance in vivo; and 3) To determine if non-phagocyte innate effectors inhibit quorum sensing-dependent virulence. These will include the paraoxonase enzyme family (thiolactonases) and the epithelial cell-expressed Nox/Duox enzymes (oxidases and peroxidases). Understanding the contribution of these innate effectors to quorum quenching and how pathogens avoid it could augment drug design that targets virulence pheromones for inactivation.